There Is No Free Lunch, Episode 763: Lens Adapters

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Lens adapters can be useful things sometimes, letting you mount one brand of lens on another brand of camera.

One thing that has always bothered me, though, is the idea of doubling the number of lens-mount interfaces. When you look at the thick metal pieces on the front of the camera and the back of the lens, and then consider that they have to be lined up exactly parallel to the image sensor, it's kind of amazing it works.

Although it doesn't always work. Lloyd Chambers first reported years ago that with high-quality, wide-angle lenses you could detect very small misalignments in the camera-lens mount. Misalignment of 10 microns from side-to-side was enough to cause blur on the sides of the image. Since then a lot of other people have confirmed the same thing.

So when I hear people cavalierly talking about putting an adapter on their camera I tend to cringe. When a single camera-lens interface has enough variability to sometimes be visible, adding another large piece of metal with another mount interface seems a recipe for problems.

Don't get me wrong. Generally, they're acceptable or people wouldn't use them. But I always am curious about what acceptable looks like in the lab.

Optical Bench Testing

We've been working a lot with our optical bench, testing large enough quantities of each lens to develop our acceptable ranges, since we plan to start adding this testing to the Imatest testing we currently use for quality assurance. An optical bench isn't necessarily better than image-based testing programs like Imatest, but it has some specific advantages.

One big difference is that an optical bench tests at infinity (on wide-angle lenses, Imatest or DxOAnalytics may be testing at 4-6 feet focusing distance). Another is it tests the lens directly; the variability of a camera body is eliminated from the loop. There are some things a bench doesn't do as well, too. For example you don't get a full picture of the entire lens in one run; you get a line of data from one side to the other. You have to rotate the lens in its mount and do several lines to get a complete picture of the entire lens surface.

Since the information from our optical bench is different from the Imatest graphs I usually use for illustrations, let me go over one quickly.


Wells Optical Bench printout of MTF by field of view and frequency.


The horizontal axis shows degrees off-center, with "0" the center of the lens. The vertical axis is the MTF reading ("1" being theoretical perfection and "0" being gray mush).

The charted colors show various frequencies. For this graph we chose to show the MTF at 10, 20, 50, and 80 line pairs per mm. Most manufacturers' MTF graphs limit themselves to 10 and 30 or 10, 20, and 40 as frequencies. We're including some higher frequencies just because we're still learning about using this tool to identify bad lenses.

There are two charted lines at each frequency, one representing tangential and the other sagittal lines. When the two lines of the same color are separated, there is some astigmatism. (You don't have to worry about the terms - just that if the two lines of the same color are widely separated, that's not good, close together is good.)

The graph above is of a good copy of a good lens, the Zeiss 35mm f/2.0. There are some slight differences away from center with one side having a bit more astigmatism and the other a bit lower MTF at higher frequencies, but this is really minor stuff that wouldn't show up in photograph.

To give you a bit more experience with this kind of graph, below are printouts from 4 other Zeiss 35mm f/2.0 lenses - all of which are optically excellent as determined by Imatest and careful pixel-peeping.



Again, let me emphasize that what you're seeing is normal (actually less than normal) copy-to-copy variation in good copies of the lens. Actual bench testing is almost like fingerprinting. No two copies are exactly the same. Notice the similarities. There is nearly no astigmatism right at the center and similar MTF values, particularly at the 10 and 20 /mm frequencies that are most critical.

To give you some idea of what a not-so-good lens looks like on the same set of parameters, here's one that's not so good.

Notice this one is still quite good in the center, but has some major problems developing on the right side. The settings on the optical bench we used for this series make it look much worse than it really is. While the graph makes it look like the MTF drops to zero, that's simply because the settings we're using report zero if focusing distance changes greatly or vignetting become severe.  That makes a nice warning signal for 'some human needs to come check this lens'.

Our parameters are pretty tight: the awful looking graph above actually is a lens that looks a little softer on the right side, but certainly not horrible. An online sized jpg would look perfectly fine, at 50% pixel-peeping or in a large print you'd notice it. I'll go into more detail about what we can do with the optical bench in some later posts; I just wanted to give you a quick overview for now.

Using Adapters on the Optical Bench

One thing you probably haven't thought about is that lenses have to be mounted on the bench in order to do these tests. That requires a separate, fairly expensive mount for each brand of lens. Obviously we had to pony up to get mounts for Canon, Nikon, NEX, and Micro 4/3 lenses. But, since I was already pretty unpopular in the accounting department, I hoped to avoid spending a few thousand more dollars to buy Leica, PL, and other mounts for lenses that we have a lower number of copies of.

I knew adapters might cause a problem, but thought, since we carry so many copies of various high-quality adapters, I could certainly find a few that were accurate enough to use. Once again, Roger's assumptions were way off base. I won't bore you with dozens and dozens of test results. But I'll show you a good example. In this case, we took the lens in the upper right of the 4 examples at the top of the page and tested it on a number of Nikon to NEX adapters. Here are 6 examples.

I won't bore you with another 20 graphs that look pretty much like these. We tried Leica to NEX and Leica to Micro 4/3 adapters, Canon to NEX, etc. We tried different lenses on one adapter. It didn't really matter. None of them would be acceptable for testing. Not one.

I'll point out that we carry only name-brand, fairly expensive adapters, not eBay $29 adapters. All of them are tested frequently and used frequently and none of the ones I tested today had any problems. Still, not one of them would be acceptable for testing, so I guess I'm going to have to order those expensive lens mounts after all.

What Does It Mean in the Real World?

Like a lot of laboratory testing, probably not a lot. Adapters couldn't all stink or people wouldn't use them. Like a lot of tests, you can detect a very real difference in the lab that doesn't make much difference at all in the real world.

Videographers are the primary users of adapters, and probably won't notice the problems at all. Video and cinema cameras shoot at lower resolution (even 4K video) than photography and tend to concentrate on center-frame so they're unlikely to see a problem.

Even photographers who use adapters are often adapting a larger format lens to a smaller format camera (Leica full-frame lens to Micro 4/3 or APS-C camera, for example). Assuming the lens is higher quality than a native lens they would otherwise be shooting, they might be perfectly happy. Still, I should point out that I  only tested these 35mm lenses out to +/- 12 degrees (their field of view is actually +/- 30 degrees). Even on a Micro 4/3 camera, the lens would have a field of view of +/- 15 degrees what we see here at 12 degrees should be noticeable.

In the examples above, though, center resolution is pretty much unchanged, it's only when you get away from center that you start to see issues. So someone shooting portraits and centered subjects is unlikely to notice an issue. A landscape photographer, though, would likely see some problems along the edges of the image.

Putting a great lens on your camera via an adapter might still be better than an average native-mount lens. On the other hand, that great lens certainly wouldn't be as good as it would be on its native-mount camera.


Roger Cicala


September, 2013

101 Responses to “There Is No Free Lunch, Episode 763: Lens Adapters”

Vrecha said:

Reading this article I began to wonder ...
Olympus is claiming that 4/3 lenses can be used via their adapter (MMF-3 for example) on their m4/3 bodies. While the newest E-M1 body promises the useful AF performance etc, it would be interested to know how much the image quality is compromised because of the adapter.
It seems to me that you posses the needed knowledge, curiosity and equipment to find the (unbiased) answer.

Don Cox said:

Depth of focus was a problem in the days of film, because film (especially 120 size film) does not lie perfectly flat in the image plane. An adapter whose faces are not perfectly parallel has the same effect as film that is not flat - parts of the image will be focussed at a different distance from the rest.

If the subject is at infinity and the lens is wide open, even tiny differences in lens-to-sensor distance can visibly affect focus.

Formulae and tables can be found in the Focal Encyclopedia of Photography. In extreme cases there are figures such as .009mm.

Do we know how accurately the sensors in cameras are placed parallel to the lens mount? What happens if the sensor has 5-axis stabilisation?

Wide aperture lenses with short focal lengths are difficult to make and to use.

LensRentals Employee

Roger Cicala said:

Guys, I'll try to repeat this with some Imatest measurements, but that's more time consuming. We can run an optical bench test in about 3 minutes, compared to a 45 minute set up and 15 minutes or so per adapter for Imatest. I need most of my spare time right now to learn my way around the optical bench since we hope to use it for routine lens testing between rentals.

SteveL said:

You havce probably done this but I wonder if Imatest can see the same difference. Test a lens with and without an adapter on the optical bench then test the same lens with and without the same adapter using Imatest.

NancyP said:

Thanks. I suppose I will see what the real world impact is, given that I just bought a used Nikon 50mm f/1.2 AIS for use on my Canon 6D. Why bother with this lens? Interesting bokeh at f/1.2, at which point there is also some loss of contrast, etc, one of those situations where the lens' defect can be turned into a feature. It is also said to be very sharp when stopped down.

Wally said:

I have not used an optical bench tester, but all the optical test systems that I have seen around the laboratory I work at are either set-up with light blocking black curtains or in windowless rooms. A bit of stray light in the lens could certainly lower the image contrast.

Ian Anderson said:

I've always wondered about this with people like Marc Adamus shooting the Nikon 14-24 on his Canon bodies with a Novoflex adapter. Obviously it works extremely well for him, but interesting to see the lab results.

Ben said:

Here's another vote for running the above combo through Imatest.

Samuel H said:

I have NEX camera, all my lenses are vintage Leitz for the Leica-R mount, I use them with either a cheap adapter from ebay or a Metabones Speedbooster, depending on the situation.
You have to look at the issue as a whole to understand why adapters can be so great: yes, these adapters, specially the cheap one from ebay, must be screwing up the glass-sensor interface; and still, these lenses are incredibly more awesome than anything I could buy today for a similar price. Like, insanely better.
Of course, I should provide proof of what I'm saying, so: sharpness and bokeh tests here:

Lasse Beyer said:

Did you perform any tests with the lights turned out yet? If it makes any difference, you could use some kind of hood to cover the whole thing. Something like the Pelican case where the SigZilla ships in comes to mind here... ;)

LensRentals Employee

Roger Cicala said:

Don, you're totally correct. 2 stops down generally makes them all look great. Like I said, this stuff is more intellectual exercise, but adapters work pretty well or people wouldn't use them.

I was looking more from the point of view of "I want to shoot a Canon 35 f/1.4 on my NEX instead of an NEX lens because the Canon is a better lens" (not claiming it is, just that I hear that kind of thing a lot). It may be better in theory than a native mount lens, but not in practice after the adapter is added.


LensRentals Employee

Roger Cicala said:


I'll run some longer lenses and see. I'm guessing there will be less difference since there's a narrower angle of view and what we've seen has been fine on-center.

Jose Nuno said:

Adding to the list of reasons to use adapters is also the fact that you can get very interesting lenses for insanely low prices.

I have two manual lenses (52mm f1.8 and 135mm f3.5) that each cost around 30€, and they work fine in a NEX system. Even more so given the fact that the biggest reason for out of focus areas is in my clumsy hastiness focusing manually, and not on the lenses' tolerances.

I get that you evaluated higher value propositions, and that this setup is not going to work on a pro system where you need better reliability, but I just felt this should be pointed out just the same.

Don Cox said:

I would like to see MTF charts for these 35mm lenses (with adapters) at f/4 and f/5.6 as opposed to wide open. My guess is that stopping down a little will increase the depth of focus enough to cover the misalignment.

And who shoots landscapes or cityscapes at f/2 ?

If you are using the lens wide open for one of those shallow depth of field portraits with just one eye in focus, then you can focus that eye correctly wherever it is in the frame. It doesn't matter if objects in some other part of the frame are focussed at a slightly different distance - usually the aim is to have them out of focus.

If you use the lens wide open for street photography at night, does it really matter if objects on the left side of the picture are in focus a few inches further away than objects on the right side ?

Don Cox said:

"I think you just convinced me not to buy into a NEX system, which I planned to use with my Nikon lenses."

I have been using a NEX with Nikon lenses with complete success for the past two years. There are absolutely no problems in practice with normal and longer lenses at normal apertures.

However, I would not use a lens shorter than about 18mm with an adapter at a wide aperture. Depth of focus diminishes with focal length.

Much of my work has been with bellows such as the Nikon PB-4, which has built in "misalignment" in the form of a swing adjustment on the front panel. The depth of focus at the sensor is plenty to cover the inaccuracies of this movement at its zero setting.

Nqina Dlamini said:

Interesting read as always. I suppose if you had old FD mounts (old Canon) and wanted to use them on EF cameras, you should have some inkling that you are in a compromised situation.
Thanks again. I enjoyed this article (as always).

PenGun said:

I has planed on getting a hold of a Leica Elmar 135 f4 M for my Fuji X-E1. Is the longer focal length going to make an adapted lens less of a compromise. I'm perfectly happy with the Fuji 14mm f2.8 for the short end.

Silvio / KH said:

I think you just convinced me not to buy into a NEX system, which I planned to use with my Nikon lenses.

Not sure if I should thank you or not ;)

brandon said:

what does Imatest say about the adapters?
Interesting as always Roger, thanks.

CarVac said:

How about less extreme adapters, like Nikon->Canon or Contax->Canon?

They're usually made out of a single piece of metal, so the variability in a high-quality one should be less than an SLR->mirrorless.

LensRentals Employee

Roger Cicala said:

DEamonius, I don't have any Leitax adapters so I can't comment on them at all.

LensRentals Employee

Roger Cicala said:


We're trying to figure that out a bit. It's very consistent with several different mounts.

One thing that has recently been suggested is that the left side of the machine is against a wall, the right side exposed to a very bright room - and the front of the lens is open to that ambient light during testing. I'm going to feel really stupid if turning out the lights makes a difference after running hundreds of tests.

But that seems a likely answer. The mounts we use are rotatable through 360 degrees. If the mounts were tilted I'd expect to be able to see that move over to the left side if we rotated the mount 180 degrees, but we don't see that at all.

It's also possible there's an alignment issue in the bench itself, but repeated checks haven't revealed it yet.

I'll also mention that our bench, while giving us some capabilities we really want, is a $50k piece of equipment, which puts it in the 'economy' price range (The best ones are 5 times that price). It has 3 separate motors controlling lens rotation, reticle movement, and focusing, which are calibrated to move together. It may be that in this price range we're going to have a bit of tilt.



Oskar Ojala said:

I think that very slight deviations from perfect planarity usually don't matter, since most real world photography isn't of brick walls. Shooting a landscape at infinity is the tricky proposition that would show issues with planarity. But many native mount lenses also fail that test, reasons including soft corners, field curvature, bad tolerances in manufacturing, inaccuracy of focus and slight misalignment of lens and camera mounts with both still within spec.

Shooting at shorter distances than infinity and more three dimensional subjects, the circumstances themselves tend to mask minor defects and things like overall contrast become more important than a slight tilt of the lens.

Now I'm not saying that this all doesn't matter, just that real-life performance and value can be a very tricky thing to assess.

Brian Leighty said:

I have the same question as James but would also through extension tubes in the mix as well since I use those a lot. Great article Roger.

Lee Saxon said:

Strange coincidence that all the Zeiss 35/2 copies had a bit more drop in MTF on the right side!

eths said:

I use (heavily) two Novoflex adapters for Nikon and Pentax lenses on my Nex7.

I haven't noticed any negative effects in everyday use. Nonetheless, your test is very interesting, indeed.

Daemonius said:

What about Leitax? Bit different than usual adapters (I have it on my C/Y 50/1.4 .. works nicely).

James Scholz said:

Once again, great article, Roger. But it makes me wonder if the same problem exists with tel extenders, which introduce additional mount surfaces to the lens-camera equation. Any thoughts on this?

Otm Shank said:

Hi Roger,

Thank you for the informative post. I have always wanted to see how results from an optical bench test would look like.

Question: Why is the off-center values significantly different between the left and right side on the first plot? It looks like the right side is systematically weaker. Does it indicate that even the native mount is slightly off-center?


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